Transponder arrangements have been used to detect and uniquely identify, in a contactless manner, objects, animals, or persons being present at selected locations. Transponder systems typically include an interrogation unit which transmits radio frequency pulses and a transponder unit which receives the pulses and responds with stored data in the form of a modulated radio frequency carrier. Because the transponders may be diminutive in size, transponder systems may be used in countless applications. For example, luggage being transported on a conveyor belt may be identified and routed according to the encoded destination at a routing point. Machine components may be identified and transported to specific stations on an assembly line. Movement and activity of animal stock with embedded transponder units may be monitored and recorded in an unobtrusive manner. Personnel may carry identification badges having a transponder unit to gain access to secured areas without having to use a card reader.
However, a problem arises when multiple transponders are simultaneously present within the inquiry field of an interrogation unit. If multiple transponder units exist in the inquiry field and responding simultaneously to the interrogation pulses of the interrogation unit, the responses may become garbled and unreadable. In particular, the resonant circuits of the transponders may interfere with the operation of other circuits so that no transponder response can be read.
Accordingly, a need has arisen for resolving the problem of detecting multiple transponders located in close proximity with one another in the inquiry field of an interrogation unit.